This invention relates to systems and methods for removing sulfur from gas streams.
Many industrial fuel gases contain sulfur compounds, such as hydrogen sulfide (H2S) and carbonyl sulfide (COS). For example, synthetic gas (syngas) typically prepared by reforming or gasifying a carbonaceous fuel via partial oxidation reactions under high temperature conditions, generally comprises H2, CO, steam, and gaseous contaminants including H2S and COS. The carbonaceous fuel may be any of various solid, liquid, or gaseous materials having a substantial elemental content of carbon and hydrogen. Such materials include, for example, coal or petroleum coke, biomass waste, liquid feedstocks such as heavy naphtha fractions, or gaseous feedstocks such as natural gas.
In recent years, substantial research and investment has been directed towards various syngas processes, such as Integrated Gasification Combined Cycle (IGCC) and Coal-to-Liquids (CTL) processes. IGCC is a process for generating syngas by gasification of solid or liquid fuels with the syngas then being used as a fuel source in a combined cycle power plant. CTL is a process that uses syngas from coal gasification as a raw material for generation of high-value chemicals or zero-sulfur diesel and gasoline as transportation fuels. Syngas can also be used to produce hydrogen for fuel cells. Although syngas-based technologies offer considerable improvement in both thermal and environmental efficiency, the cost of these technologies is currently impeding market penetration.
The sulfur removal process represents a significant portion of capital costs associated with IGCC plants, CTL and coal to hydrogen plants, and other plants that require removal of sulfur compounds from syngas. Three types of sulfur removal technologies have been suggested: solvent-based processes, sorbent-based processes, and direct oxidation processes. Direct oxidation processes have attempted to use air or oxygen to oxidize H2S directly to elemental sulfur in a syngas stream, but no successful example has been reported for an actual industrial application. In order to remove sulfur using liquid solvents, syngas exiting the gasifier is usually cooled to room temperatures through multiple stages which are energy and capital cost intensive. Solid sorbents, such as ZnO or MnO based sorbents are used to remove H2S in commercial scale hydrogen production from natural gas. Since the sulfur level is low in natural gas and ZnO is relatively inexpensive, regeneration of the adsorption material is not critical in such applications. Solid sorbents can also be used to remove sulfur from coal-derived syngas at warm temperatures (250° C.˜600° C.). However, it is generally required to regenerate solid sorbents in such applications due to the high sulfur content in coal syngas.
Regeneration of sulfur adsorption material usually produces sulfur dioxide, which needs to be further processed using additional equipment, thereby complicating and increasing the cost of the whole system.
Accordingly, there is a need for a feasible process to remove hydrogen sulfide from gas streams while minimizing loss of thermal efficiency and process complexity.